FIG. 1A depicts a portion of a prior art reflective (i.e. front-lit) electrophoretically frustrated total internal reflection (TIR) modulated display 10 of the type described in U.S. Pat. Nos. 6,885,496; 6,891,658; 7,164,536 and 8,040,591. Display 10 includes a transparent outward sheet 12 formed by partially embedding a large plurality of high refractive index (e.g. η1>˜1.90) transparent spherical or approximately spherical beads 14 in the inward surface of a high refractive index (e.g. η1>˜1.75) polymeric material 16 having a flat outward viewing surface 17 which viewer V observes through an angular range of viewing directions Y. The “inward” and “outward” directions are indicated by double-headed arrow Z. Beads 14 are packed closely together to form an inwardly projecting monolayer 18 having a thickness approximately equal to the diameter of one of beads 14. Ideally, each one of beads 14 touches all of the beads immediately adjacent to that one bead. Minimal interstitial gaps (ideally, no gaps) remain between adjacent beads.
An electrophoresis medium 20 is maintained adjacent the portions of beads 14 which protrude inwardly from material 16 by containment of medium 20 within a reservoir 22 defined by lower sheet 24. An inert, low refractive index (i.e. less than about 1.35), low viscosity, electrically insulating liquid such as Fluorinert™ perfluorinated hydrocarbon liquid (η1˜1.27) available from 3M, St. Paul, Minn. is a suitable electrophoresis medium. Other liquids, or water can also be used as electrophoresis medium 20. A bead:liquid TIR interface is thus formed. Medium 20 contains a finely dispersed suspension of light scattering and/or absorptive particles 26 such as pigments, dyed or otherwise scattering/absorptive silica or latex particles, etc. Sheet 24's optical characteristics are relatively unimportant: sheet 24 need only form a reservoir for containment of electrophoresis medium 20 and particles 26, and serve as a support for backplane electrode 48.
As is well known, the TIR interface between two media having different refractive indices is characterized by a critical angle θc. Light rays incident upon the interface at angles less than θc are transmitted through the interface. Light rays incident upon the interface at angles greater than θc undergo TIR at the interface. A small critical angle is preferred at the TIR interface since this affords a large range of angles over which TIR may occur.
In the absence of electrophoretic activity, as is illustrated to the right of dashed line 28 in FIG. 1A, a substantial fraction of the light rays passing through sheet 12 and beads 14 undergoes TIR at the inward side of beads 14. For example, incident light rays 30, 32 are refracted through material 16 and beads 14. The rays undergo TIR two or more times at the bead:liquid TIR interface, as indicated at points 34, 36 in the case of ray 30; and indicated at points 38, 40 in the case of ray 32. The totally internally reflected rays are then refracted back through beads 14 and material 16 and emerge as rays 42, 44 respectively, achieving a “white” appearance in each reflection region or pixel.
A voltage can be applied across medium 20 via electrodes 46 and 48 which can for example be applied by vapour-deposition to the inwardly protruding surface portion of beads 14 and to the outward surface of sheet 24. Electrode 46 is transparent and substantially thin to minimize its interference with light rays at the bead:liquid TIR interface. Backplane electrode 48 need not be transparent. If electrophoresis medium 20 is activated by actuating voltage source 50 to apply a voltage between electrodes 46 and 48 as illustrated to the left of dashed line 28, suspended particles 26 are electrophoretically moved adjacent the surface of the monolayer of beads 18 into the region where the evanescent wave is relatively intense (i.e. within 0.25 micron of the inward surfaces of inwardly protruding beads 14, or closer). When electrophoretically moved as aforesaid, particles 26 scatter or absorb light, thus frustrating or modulating TIR by modifying the imaginary and possibly the real component of the effective refractive index at the bead:liquid TIR interface. This is illustrated by light rays 52 and 54 which are scattered and/or absorbed as they strike particles 26 inside the thin (˜0.5 μm) evanescent wave region at the bead:liquid TIR interface, as indicated at 56 and 58 respectively, thus achieving a “dark” appearance in each TIR-frustrated non-reflective absorption region or pixel. Particles 26 need only be moved outside the thin evanescent wave region, by suitably actuating voltage source 50, in order to restore the TIR capability of the bead:liquid TIR interface and convert each “dark” non-reflective absorption region or pixel to a “white” reflection region or pixel.
As described above, the net optical characteristics of outward sheet 12 can be controlled by controlling the voltage applied across medium 20 via electrodes 46 and 48. The electrodes can be segmented to control the electrophoretic activation of medium 20 across separate regions or pixels of sheet 12, thus forming an image.
FIG. 2 depicts, in enlarged cross-section, an inward hemispherical or “hemi-bead” portion 60 of one of spherical beads 14. Hemi-bead 60 has a normalized radius r=1 and a refractive index η1. A light ray 62 perpendicularly incident (through material 16) on hemi-bead 60 at a radial distance a from hemi-bead 60's center C encounters the inward surface of hemi-bead 60 at an angle θ1 relative to radial axis 66. For purposes of this theoretically ideal discussion, it is assumed that material 16 has the same refractive index as hemi-bead 60 (i.e. η1=η2), so ray 62 passes from material 16 into hemi-bead 60 without refraction. Ray 62 is refracted at the inward surface of hemi-bead 60 and passes into electrophoretic medium 20 as ray 64 at an angle θ2 relative to radial axis 66.
Now consider incident light ray 68 which is perpendicularly incident (through material 16) on hemi-bead 60 at a distance
      a    c    =            η      3              η      1      from hemi-bead 60's center C. Ray 68 encounters the inward surface of hemi-bead 60 at the critical angle θc (relative to radial axis 70), the minimum required angle for TIR to occur. Ray 68 is accordingly totally internally reflected, as ray 72, which again encounters the inward surface of hemi-bead 60 at the critical angle θc. Ray 72 is accordingly totally internally reflected, as ray 74, which also encounters the inward surface of hemi-bead 60 at the critical angle θc. Ray 74 is accordingly totally internally reflected, as ray 76, which passes perpendicularly through hemi-bead 60 into the embedded portion of bead 14 and into material 16. Ray 68 is thus reflected back as ray 76 in a direction approximately opposite that of incident ray 68.
All light rays which are incident on hemi-bead 60 at distances a≥ac from hemi-bead 60's center C are reflected back (but not exactly retro-reflected) toward the light source; which means that the reflection is enhanced when the light source is overhead and slightly behind the viewer, and that the reflected light has a diffuse characteristic giving it a white appearance, which is desirable in reflective display applications. FIGS. 3A, 3B and 3C depict three of hemi-bead 60's reflection modes. These and other modes coexist, but it is useful to discuss each mode separately.
In FIG. 3A, light rays incident within a range of distances ac<a≤a1 undergo TIR twice (the 2-TIR mode) and the reflected rays diverge within a comparatively wide arc φ1 centered on a direction opposite to the direction of the incident light rays. In FIG. 3B, light rays incident within a range of distances a1<a≤a2 undergo TIR three times (the 3-TIR mode) and the reflected rays diverge within a narrower arc φ2<φ1 which is again centered on a direction opposite to the direction of the incident light rays. In FIG. 3C, light rays incident within a range of distances a2<a≤a3 undergo TIR four times (the 4-TIR mode) and the reflected rays diverge within a still narrower arc φ3<φ2 also centered on a direction opposite to the direction of the incident light rays. Hemi-bead 60 thus has a “semi-retro-reflective,” partially diffuse reflection characteristic, causing display 10 to have a diffuse appearance akin to that of paper.
Display 10 has relatively high apparent brightness, comparable to that of paper, when the dominant source of illumination is behind the viewer, within a small angular range. This is illustrated in FIG. 1B which depicts the wide angular range a over which viewer V is able to view display 10, and the angle β which is the angular deviation of illumination source S relative to the location of viewer V. Display 10's high apparent brightness is maintained as long as β is not too large. At normal incidence, the reflectance R of hemi-bead 60 (i.e. the fraction of light rays incident on hemi-bead 60 that reflect by TIR) is given by equation (1):
                    R        =                  1          -                                    (                                                η                  3                                                  η                  1                                            )                        2                                              (        1        )            where η1 is the refractive index of hemi-bead 60 and η3 is the refractive index of the medium adjacent the surface of hemi-bead 60 at which TIR occurs. Thus, if hemi-bead 60 is formed of a lower refractive index material such as polycarbonate (η1˜1.59) and if the adjacent medium is Fluorinert (η3˜1.27), a reflectance R of about 36% is attained, whereas if hemi-bead 60 is formed of a high refractive index nano-composite material (η1˜1.92) a reflectance R of about 56% is attained. When illumination source S (FIG. 1B) is positioned behind viewer V's head, the apparent brightness of display 10 is further enhanced by the aforementioned semi-retro-reflective characteristic.
As shown in FIGS. 4A-4G, hemi-bead 60's reflectance is maintained over a broad range of incidence angles, thus enhancing display 10's wide angular viewing characteristic and its apparent brightness. For example, FIG. 4A shows hemi-bead 60 as seen from perpendicular incidence—that is, from an incidence angle offset 0° from the perpendicular. In this case, the portion 80 of hemi-bead 60 for which a≥ac appears as an annulus. The annulus is depicted as white, corresponding to the fact that this is the region of hemi-bead 60 which reflects incident light rays by TIR, as aforesaid. The annulus surrounds a circular region 82 which is depicted as dark, corresponding to the fact that this is the non-reflective region of hemi-bead 60 within which incident rays are absorbed and do not undergo TIR. FIGS. 4B-4G show hemi-bead 60 as seen from incident angles which are respectively offset 15°, 30°, 45°, 60°, 75° and 90° from the perpendicular. Comparison of FIGS. 4B-4G with FIG. 4A reveals that the observed area of reflective portion 80 of hemi-bead 60 for which a≥ac decreases only gradually as the incidence angle increases. Even at near glancing incidence angles (e.g. FIG. 4F) an observer will still see a substantial part of reflective portion 80, thus giving display 10 a wide angular viewing range over which high apparent brightness is maintained.
The reflective, white annular region 80 surrounding the non-reflective, dark circular region 82 presents a problem commonly referred to as the “dark pupil” problem which reduces the reflectance of the display. The display's performance is further reduced by transparent electrode 46, which may be formed by provision of a transparent conductive coating on hemi-beads 14. Such coatings typically absorb about 5% to 10% of the incident light. Since a light ray typically reflects several times, this can make it difficult to achieve efficient reflection. A further related problem is that it can be expensive and challenging to apply such a coating to a contoured hemispherical surface.
The dark pupil problem can be addressed by reflecting back toward hemi-beads 14 (i.e. “recycling”) light rays which pass through the non-reflective, dark circular region 82 of any of hemi-beads 14. An approach to solving this problem and enhancing the brightness of the display is to equip the display with a reflective component to reflect the light back through the pupil and towards the viewer. FIG. 5 depicts a prior art reflective (i.e. front-lit) frustrated total internal reflection (TIR) modulated display 100 of the type described in PCT Application No. WO 2006/108285 A1 and South Korean patent No. 10-2007-7026347. Display 100 includes a transparent outer sheet 102 formed by partially embedding a large plurality of high refractive index transparent spherical or approximately spherical beads 104 in the inward surface of a high refractive index polymeric material 102 having a flat outward viewing surface 106 by which a viewer views the display image. On the surface of the plurality of spherical beads 104 is a substantially transparent electrode 107.
An electrophoresis medium 108 is contained within the reservoir or cavity formed between the portions of beads 104 which protrude inwardly from material 102 and the lower or rear sheet 110. On the inward surface of the rear sheet 110 is an electrode layer 111. The medium 108 is an inert, low refractive index, low viscosity liquid such as a fluorinated hydrocarbon. Other liquids may also be used as electrophoresis medium 108. A bead:liquid TIR interface is thus formed. Medium 108 further contains a finely dispersed suspension of light absorbing, electrophoretically mobile particles 112.
FIG. 5 further depicts a prior art reflective, continuous, porous, membrane 118 disposed between the inward surfaces of hemisphere beads 116 and rear sheet 110 to enhance the brightness of the TIR display. The average diameter of the pores in the continuous membrane 118 is substantially greater (e.g. about 10 times greater) than the average diameter of absorptive particles 112. The pores in membrane 118 constitute a sufficiently large fraction (e.g. at least 20%) of the total surface area of membrane 118 to substantially permit unimpeded passage of absorptive particles 112 through membrane 118. Membrane 118's outward surface 120 is highly reflective, and may be either diffusely or specularly reflective.
In the absence of electrophoretic activity, as is illustrated to the left of dashed line 114 in FIG. 5 prior art, the smaller absorptive particles 112 tend to settle through membrane 140's pores, toward the rear electrode layer 111 on rear sheet 110 or if the electrophoretic particles are driven to the rear electrode 111 under the influence of an applied electric field. Reflectance is thus increased and enhanced, since incident light rays (e.g. ray 122) which would otherwise have passed through the “dark pupil” region of the hemisphere beads 116 and would have been absorbed by, such as, the absorptive particles 112 located at the lower sheet 110, are instead reflected (e.g. ray 124) by membrane 118's reflective outward surface 120. Light rays (e.g. ray 126) which are incident upon reflective annular regions of the hemisphere beads are totally internally reflected (e.g. ray 128).
When a voltage is applied across medium 108, as is illustrated to the right of dashed line 114 in the prior art depicted in FIG. 5, absorptive particles 112 are electrophoretically moved through membrane 118's pores and attracted to electrode layer 107 on the inward surfaces of hemisphere beads 116. When so moved into this absorptive state, particles 112 absorb light rays (e.g. ray 130) which are incident upon the annular regions of the hemisphere beads by frustrating or modulating TIR, and also absorb light rays (e.g. rays 132) which do not undergo TIR and which would otherwise pass through beads 104. Membrane 118's pores allow absorptive particles 112 to move outwardly into contact with the surface of hemisphere beads 116 in the absorptive state; and to move inwardly away from hemisphere beads 116 in the reflective state, thus obscuring absorptive particles 112 from direct view in the reflective state.
As described in the preceding paragraphs, the porous reflective membrane's purpose is to reflect the light that passes through the dark pupil of the hemi-beads 116 back through the pupil region of the hemi-beads 116 and towards the viewer to enhance the brightness of the display as depicted in FIG. 5. An alternative porous reflective membrane structure is disclosed herein. This disclosure pertains to a TIR-based display comprising of a frustratable (i.e. the degree to which the incident light undergoes total internal reflection can be controllably modified by altering the optical properties, and specifically the refractive index values, at or near the interface at which the light undergoes TIR), optically transparent, hemispherically-contoured front sheet and a multi-functional porous continuous reflective membrane. A hemispherically-contoured front sheet is a substantially flat sheet that has a plurality of protruding surface features, said surface features having a substantially hemispherical shape. Herein, a porous continuous membrane is further comprised of a top electrically conductive metal layer that faces the hemispherical contoured surface of the optically transparent front sheet. The top metal layer is porous and continuous and acts as both a light reflector to enhance the brightness of the display and as the top electrode layer. This new structure disclosed herein simplifies manufacturing of TIR-based displays described herein and greatly lowers manufacturing costs. Additionally, by removing the mostly transparent electrode layer from the surface of the hemispherical beads as shown in the prior art display in FIG. 5, less light will be lost due to absorption by the electrode layer thus further increasing the brightness of the display. ITO can degrade over time under various conditions such as exposure to high voltages, temperatures and contact with the electrophoretic medium. As ITO degrades it becomes less transparent and further absorbs light leading to a further decrease in brightness of the display. Furthermore, removing the ITO layer and replacing with a metal layer as the top electrode reduces the resistance leading to increased switching uniformity.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.